What is the X in X-mas?
1540s, from Middle 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" (cf. robigo "rust"), from PIE *reudh- (see red (adj.1)). Related: Robustly; robustness. Robustious (1540s) was a common form in 17c. (cf. "Hamlet" iii.2); it fell from use by mid-18c., but was somewhat revived by mid-19c. antiquarian writers.
Said of a system that has demonstrated an ability to recover gracefully from the whole range of exceptional inputs and situations in a given environment. One step below bulletproof. Carries the additional connotation of elegance in addition to just careful attention to detail. Compare smart, opposite: brittle.