consistent (adj.)

1570s, "consisting" (a sense now obsolete), from Latin consistentem (nominative consistens), present participle of consistere "to stand firm, take a standing position, stop, halt," from assimilated form of com "with, together" (see con-) + sistere "to place," causative of stare "to stand, be standing," from PIE root *sta- "to stand, make or be firm."

Sense of "standing together in agreement, agreeing" (with with) is first attested 1640s; meaning "marked by consistency" is from 1732. The literal, physical sense survives in consistency. Related: Consistently.