1540s, of persons, "having or indicating great strength, muscular, vigorous," from French robuste (14c.) and directly from Latin robustus "strong and hardy," literally "as strong as oak," originally "oaken," from robur, robus "hard timber, strength," also "a special kind of oak," named for its reddish heartwood, from Latin ruber "red" (related to robigo "rust"), from PIE root *reudh- "red, ruddy." Related: Robustly; robustness; robusticity.
Robustious (1540s) was an elaborated form common in 17c. (see "Hamlet" iii.2), with more of a sense of "rough, violent, rude;" according to OED it fell from use by mid-18c., but was somewhat revived by mid-19c. antiquarian writers. Related: Robustiously; robustiousness.