Other forms: clogged, clogging, clogs
To clog is to block a pipe or an opening so that nothing can pass. Slowly, by things building up.
The gradual quality is essential, since a drain clogs over months and a blockage happens at once. It takes up, so clogging up is really the whole verb in speech. Roads and arteries clog in exactly the same way. There is a separate noun, since a clog is a wooden shoe, and clog dancing is a real tradition in the north of England and in the Netherlands.
Proto-Germanic
*klumpô — lump, mass, clasp
Middle English
clog — weight attached to the leg of an animal to impede movement
English, today
clog
unknown; perhaps from Middle English clog (“weight attached to the leg of an animal to impede movement”). Perhaps of gmq - origin and derived from Proto-Germanic klumpô (“lump, mass, clasp”); compare Old Norse klugu, klogo (“knotty tree log”), klomp.
Etymology from Wiktionary, CC BY-SA