Obstruct is Latin with the -ct- cluster English liked enough to keep: ob, "in front of, in the way of," plus struere, "to pile, to build." To obstruct is literally to build something in the way. The same struere gives construct, instruct, structure and destroy, all of them different things to do with piling material up.
English formed the word in the 1610s, either as a back-formation from the older obstruction or directly from the Latin past participle obstructus. The physical sense of blocking something up came first; the figurative one — obstructing justice, obstructing the law — followed within thirty years, by the 1640s. The prefix ob- belongs to the set of Latin prepositions that mark a word as a learned borrowing: ob-, ab-, sub-, trans-, inter-, ex-, pre-, post-.
obstruct (v.) 1610s, "to block or stop up with obstacles or impediments," a back-formation from obstruction or else from Latin obstructus , past participle 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 sense of "to hinder, impede, retard, delay" (justice, the law, etc.) is by 1640s. Related: Obstructed ; obstructing . also from 1610s
obstruct came in through Latin, and its spelling follows that family's conventions.
How that family writes these sounds
| a prefix you can hear on the front | pre-, post-, sub-, trans-, inter-, ex-, re-, de-, ab-, ob-predict, transmit, interject, obstruct |
What gives it away on the page
| pre-, post-, sub-, trans-, inter-, ex-, re-, de-, ab-, ob- | Latin prepositions doing duty as prefixes.predict, transmit, interject, obstruct |
Placing it without the spelling
| three or more syllables, stress in the middle, audibly assembled from parts | Latinimmediate, obstruct, various |