obstruct (v.)

"action of blocking up a way or passage, act of impeding passage or movement; fact of being obstructed," 1530s, from Latin obstructionem (nominative obstructio) "an obstruction, barrier, a building up," noun of action from past-participle stem of obstruere "build up, block, block up, build against, stop, bar, hinder," from ob "in front of, in the way of" (see ob-) + struere "to pile, build" (from PIE *streu-, extended form of root *stere- "to spread"). Figurative use is by 1650s.

1650s, from un- (1) "not" + past participle of obstruct (v.).