Origin and history of ostensibly

1730, "capable of being shown, that can be shown or seen, presentable," from French ostensible, from Latin ostens-, past-participle stem of ostendere "to show, expose to view; to stretch out, spread before; exhibit, display," from assimilated form of ob "in front of" (see ob-) + tendere "to stretch" (from PIE root *ten- "to stretch"). Meaning "apparent, professed, put forth or held out as real" is from 1771.

Ostensible is, literally, that may be or is held out as true, real, actual, or intended, but may or may not be so: thus, a person's ostensible motive for some action is the motive that appears to the observer, and is held out to him as the real motive, which it may or may not be. [Century Dictionary, 1895]

common adverbial suffix, forming, from adjectives, adverbs signifying "in a manner denoted by" the adjective; Middle English -li, from Old English -lice, from Proto-Germanic *-liko- (source also of Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko). See -ly (1). It is cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.