'In' is derived from "innan"—"to reside',1 "habitare", "to dwell" [sich auf halten]. 'An' signifies "I am accustomed", "I am familiar with", "I look after something".2 It has the signification of"colo" in the senses of "habito" and "diligo". The entity to which Being-in in this signification belongs is one which we have characterized as that entity which in each case I myself am (bin]. The expression 'bin' is connected with 'bei', and so 'ich bin' ['I am'] means in its turn "I reside" or "dwell alongside" the world, as that which is familiar to me in such and such a way.3 "Being" [Sein], as the infinitive of 'ich bin' (that is to say, when it is understood as an existentiale), signifies "to reside alongside ...", "to be familiar with ...". "Being-in" is thus the formal existential expression for the Being of Dasein, which has Being-in-the-world as its essential state.
'Being alongside' the world in the sense of being absorbed in the world' (a sense which calls for still closer interpretation) is an existentiale founded upon Being-in.
1 Reading 'innan—wohnen'. As Heidegger points out in his footnote, this puzzling passage has its source in Grimm's Kleinere Schriften, Vol. VII, pp. 247 ff., where we find two short articles, the first entitled 'IN' and the second 'IN UND BEI'. The first article begins by comparing a number of archaic German words meaning 'domus', all having a form similar to our English 'inn', which Grimm mentions. He goes on ·to postulate 'a strong verb "innan", which must have meant either "habitare", "domi esse", or "recipere in domum"' {though only a weak derivative form 'innian' is actually found), with a surviving strong preterite written either as 'an' or as 'ann'. Grimm goes on to argue that the preposition 'in' is derived from the verb, rather than the verb from the preposition.
2 '... "an" bedeutet: ich bin gewohnt, vertraut mit, ich pflege etwas ...'
In Grimm's second article he adds: 'there was also an anomalous "ann" with the plural "unnum", which expressed "amo", "diligo", "faveo", and to which our "gonnen" and "Gunst" are immediately related, as has long been recognized. "Ann" really means "ich bin eingewohnt", "pflege zu bauen"; this conceptual transition may be shown with minimal complication in the Latin "colo", which stands for "habito" as well as "diligo".'
It is not entirely clear whether Heidegger's discussion of 'an' is aimed to elucidate the preposition 'an' ( which corresponds in some of its usages to the English 'at', and which he has just used in remarking that the water and the glass are both at a location), or rather to explain the preterite 'an' of 'innan'.
The reader should note that while the verb 'wohnen' normally means 'to reside' or 'to dwell', the expression 'ich bin gewohnt' means 'I am accustomed to', and 'ich bin eingewohnt' means 'I have become accustomed to the place where I reside—to my surroundings'. Similarly 'ich pflege etwas' may mean either 'I am accustomed to do something' or 'I take care of something' or 'I devote myself to it'. ( Grimm's 'pflege zu bauen' presumably means 'I am accustomed to putting my trust in something', 'I can build on it'.) The Latin, 'colo' has the parallel meanings of 'I take care of something' or 'cherish' it ('diligo') and 'I dwell' or 'I inhabit' ('habito').
3 '... ich wohne, halte mich auf bei ... der Welt, als dem so und so Vertrauten.' The preposition 'bei', like 'an', does not have quite the semantical range of any English preposition. Our 'alongside', with which we shall translate it when other devices seem less satisfactory, especially in the phrase 'Being alongside' ('Sein bei'), is often quite misleading; the sense here is closer to that of 'at' in such expressions as 'at home' or 'at my father's', or that of the French 'chez'. Here again Heidegger seems to be relying upon Grimm, who proceeds (loc. cit.) to connect 'bei' with 'bauen' ('build') and 'bin'.
4 '... in dem ... Sinne des Aufgehens in der Welt ...' 'Aufgehen' means literally 'to go up', or 'to rise' in the sense that the sun 'rises' or the dough 'rises'. But when followed by the preposition 'in', it takes on other meanings. Thus 5 'geht auf' into go in the sense that it 'goes into' 30 without remainder; a country 'geht auf' into another country into which it is taken over or absorbed; a person 'geht auf' in anything to which he devotes himself fully, whether an activity or another person. We shall usually translate 'aufgehen' by some form of 'absorb'.