A lot vs. Alot: 9 Grammatical Pitfalls
1650s, from un- (1) "not" + past participle of obstruct.
1610s, a back-formation from obstruction or else from Latin obstructus, past participle of obstruere "to block, to stop up" (see obstruction). Related: Obstructed; obstructing.
obstruct ob·struct (əb-strŭkt', ŏb-)v. ob·struct·ed, ob·struct·ing, ob·structs To block or close a body passage so as to hinder or interrupt a flow.