underwrite (v.)

early 15c., subscriben, "to sign at the bottom of a document" (a sense now rare); mid-15c., "give one's consent, bind oneself" (by subscribing one's name); from Latin subscribere "write, write underneath, sign one's name; register," also figuratively "assent, agree to, approve," from sub "underneath" (see sub-) + scribere "to write" (from PIE root *skribh- "to cut").

The sense of "give one's assent" (to) an opinion, theory, etc., is by 1540s. The meaning "promise to contribute money to" is by 1630s; and that of "become a regular buyer of (a publication)" 1711; both originally literal. Related: Subscribed; subscribing.

Old English under (prep.) "beneath, among, before, in the presence of, in subjection to, under the rule of, by means of," also, as an adverb, "beneath, below, underneath," expressing position with reference to that which is above, from Proto-Germanic *under- (source also of Old Frisian under, Dutch onder, Old High German untar, German unter, Old Norse undir, Gothic undar), from PIE *ndher- "under" (source also of Sanskrit adhah "below;" Avestan athara- "lower;" Latin infernus "lower," infra "below").

Productive as a prefix in Old English, as in German and Scandinavian (often forming words modeled on Latin ones in sub-). Notion of "inferior in rank, position, etc." was present in Old English. With reference to standards, "less than in age, price, value," etc., late 14c. As an adjective, "lower in position; lower in rank or degree" from 13c. Also used in Old English as a preposition meaning "between, among," as still in under these circumstances, etc. (though this may be an entirely separate root; see understand).

Under the weather "indisposed" is from 1810. To keep something under (one's) hat "secret" is from 1885; to have something under (one's) nose "in plain sight" is from 1540s; to speak under (one's) breath "in a low voice" is attested from 1832.

The adjectival phrase under-the-table "hidden from view" (often implying "illicit") is recorded from 1949 (under-board "dishonest" is from c. 1600); to be under the table "passed out from excess drinking" is recorded from 1913.