ado n.

Old English æt, from Proto-Germanic *at (source also of Old Norse, Gothic at, Old Frisian et, Old High German az), from PIE root *ad- "to, near, at." Lost in German and Dutch, which use their equivalent of to; in Scandinavian, however, to has been lost and at fills its place.

At is used to denote relations of so many kinds, and some of these so remote from its primary local sense, that a classification of its uses is very difficult. [OED]

In choosing between at church, in church, etc. at is properly distinguished from in or on by involving some practical connection; a worshipper is at church; a tourist is in the church. In 19c. it was used for points of the compass as regions of a country (at the South) where later tendency was to use in.

The colloquial use of at after where (as in where it's at) is noted in Bartlett (1859). At last is recorded from late 13c.; adverbial phrase at least was in use by 1775. At in Middle English was used freely with prepositions (as in at after, which is in Shakespeare), but this has faded with the exception of at about.

"perform, execute, achieve, carry out, bring to pass by procedure of any kind," etc., Middle English do, first person singular of Old English don "make, act, perform, cause; to put, to place," from West Germanic *doanan (source also of Old Saxon duan, Old Frisian dwa, Dutch doen, Old High German tuon, German tun), from PIE root *dhe- "to set, put, place."

Use as an auxiliary began in Middle English. Sense of "to put, place, lay" is obsolete except in phrases such as do away with. Periphrastic form in negative sentences (They did not think) replaced the Old English negative particles (Hie ne wendon).

Meaning "visit as a tourist" is from 1817. In old slang it meant "to hoax, cheat, swindle" (1640s). Slang meaning "to do the sex act with or to" is from 1913.

Slang do in "bring disaster upon, kill" is by 1905. To have to do with "have concern or connection with" is from late 13c. To do without "dispense with" is from 1713.  Expression do or die indicating determination to succeed despite dangers or obstacles is attested from 1620s.

Compare does, did, done.