"building whose rafters lean against another building or wall," mid-15c., from lean (v.) + to (adv.). Compare penthouse. "An addition made to a house behind, or at the end of it, chiefly for domestic offices, of one story or more, lower than the main building, and the roof of it leaning against the wall of the house" [Bartlett].
Entries linking to lean-to
Transitive sense "cause to lean or rest" is from 14c. Meaning "to incline the body against something for support" is mid-13c. Figurative sense of "to trust for support" is from early 13c. Sense of "to lean toward mentally, to favor" is from late 14c. Related: Leaned; leaning. Colloquial lean on "put pressure on" (someone) is first recorded 1960.
Old English to "in the direction of, for the purpose of, furthermore," from West Germanic *to (source also of Old Saxon and Old Frisian to, Dutch toe, Old High German zuo, German zu "to"), from PIE pronominal base *do- "to, toward, upward" (source also of Latin donec "as long as," Old Church Slavonic do "as far as, to," Greek suffix -de "to, toward," Old Irish do, Lithuanian da-), from demonstrative *de-. Not found in Scandinavian, where the equivalent of till (prep.) is used.
The nearly universal use of to with infinitives (to sleep, to dream, etc.) arose in Middle English out of the Old English dative use of to, and it helped drive out the Old English inflectional endings (though in this use to itself is a mere sign, without meaning).
Commonly used as a prefix in Middle English (to-hear "listen to," etc.), but few of these survive (to-do, together, and time references such as today, tonight, tomorrow — Chaucer also has to-yeere). To and fro "side to side" is attested from mid-14c. Phrase what's it to you "how does that concern you?" (1819) is a modern form of an old question:
Huæd is ðec ðæs?
[John xxi:22, in Lindisfarne Gospel, c.950]
c. 1300, pentis, pendize, "a shed or sloping roof projecting from a main wall or the side or end of a building," from Anglo-French pentiz, a shortening of Old French apentis "attached building, appendage," from Medieval Latin appendicium, from Latin appendere "to hang" (see append).
The modern spelling is from c. 1530 by folk etymology influence of French pente "slope," and English house (the meaning at that time was "attached building with a sloping roof or awning"). Originally a simple structure (Middle English homilies describe the stable where Jesus was born as a "penthouse"); meaning "apartment or small house built on the roof of a skyscraper" is attested by 1921, from which time dates its association with luxury.