1794, from under + carriage (n.). Meaning "landing gear of an aircraft" is recorded from 1911.
Entries linking to undercarriage
under prep., adv.
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. Under the table is from 1913 in the sense of "very drunk," 1940s in sense of "illegal" (under-board "dishonest" is from c. 1600). 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.
carriage n.
late 14c., "act of carrying, means of conveyance; wheeled vehicles collectively," from Anglo-French and Old North French cariage "cart, carriage, action of transporting in a vehicle" (Old French charriage, Modern French charriage), from carier "to carry," from Late Latin carricare, from Latin carrus "two-wheeled wagon" (see car).
The meaning "individual wheeled vehicle" is c. 1400; the specific sense of "horse-drawn, wheeled vehicle for hauling people" is attested from 1706; extended to railway cars by 1830. The meaning "the business of transportation" is from 1520s. The meaning "way of carrying one's body" is by 1590s, hence also "behavior, conduct, manners." The sense of "a part of a machinery which carries another part" is from 1680s. Carriage-house is attested from 1761.