'Tenable' is Latin for 'able to be held' — a fortress that withstands siege, an argument that withstands criticism.
Able to be maintained or defended against attack or objection; (of a position or scholarship) able to be held or occupied.
From Old French tenable (capable of being held, defensible), from tenir (to hold), from Latin tenēre (to hold, to keep). Tenēre derives from Proto-Indo-European *ten- (to stretch, to hold taut), a root with extensive reflexes: Latin tenuis (thin, stretched out), tendere (to stretch), tenax (holding fast), tensus (stretched). Greek teinein (τείνειν, to stretch) and Sanskrit tanoti (he stretches) confirm the PIE inheritance. The English adjective tenable entered in the 16th century with the concrete
The military sense came first: a tenable fortress could be held against siege. The intellectual sense — a tenable argument, a tenable theory — is a metaphorical extension from warfare to debate. We still speak of 'defending' a thesis, 'attacking' an argument, and finding a position 'untenable' — all military metaphors applied to intellectual combat. The vocabulary of academic debate is essentially the vocabulary of siege warfare.