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ostensible [ostensible 词源字典]: ostensible: [18] Ostensible means literally ‘that can be shown’. It comes via French from medieval Latin ostensibilis, a derivative of the Latin verb ostendere ‘show’ (itself a compound formed from the prefix ob- ‘in front of’ and tendere ‘stretch’, source of English extend, tend, tense etc). Its original meaning ‘showable’ survived into English (‘You should send me two letters – one confidential, another ostensible’, Jeremy Bentham, 1828), but seems to have died out by the mid-19th century.

Two metaphorical strands came with it, though. One, ‘vainly conspicuous’, goes right back to ostendere, and is still preserved in English ostentation [15], although it has disappeared as far as ostensible is concerned. The other, ‘presented as real but not so’, is today the central meaning of the adjective.
=> extend, ostentation, tend, tense[ostensible etymology, ostensible origin, 英语词源]
extensible (n.): 1610s, from French extensible, from stem of Latin extendere (see extend). Earlier was extendible (late 15c.).
ostensible (adj.): 1762, "capable of being shown, 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 ob "in front of" (see ob-) + tendere "to stretch" (see tenet). Meaning "apparent, professed" is from 1771.