What does caulk mean?
Caulk is a waterproof material—typically silicone, latex, or polyurethane based—used to seal joints and gaps where two surfaces meet, preventing water, air, and dust from passing through. It is most familiar in domestic settings: sealing bathtubs, sinks, window frames, and tile grout lines. The word also functions as a verb, meaning to apply this sealant, as in 'to caulk the windows before winter.' Historically, caulking was crucial to shipbuilding: craftsmen drove tarred hemp fibre known as oakum into the seams between wooden planks to keep vessels watertight, a practice dating back centuries. The term derives ultimately from Latin 'calx,' meaning limestone or chalk, reflecting the mineral-based compounds once used as fillers. Today caulk is an essential word in construction, DIY, and plumbing vocabulary, associated with practicality and maintenance rather than formality.
A waterproof sealant or filler used to close joints and seams against leakage.
"He squeezed a line of white caulk into the gap beneath the window frame."
Often used attributively, as in 'caulk gun' or 'caulk tube.'
To seal a joint, seam, or gap with caulk.
"Remember to caulk around the sink before you tile."
Regular verb: caulk, caulked, caulking.
Rarely pluralised; usually treated as an uncountable mass noun. A plural may appear informally when distinguishing different types of caulk.
"The hardware store stocks several different caulks for indoor and outdoor use."
Before modern silicone tubes, shipwrights hammered oakum—tarred rope fibres—into hull seams by hand; the word 'caulk' literally kept wooden ships afloat for centuries.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of caulk
Caulk entered English in the late Middle Ages, likely borrowed from Old Northern French 'cauquer' (to trample or press), which itself derives from Latin 'calcare,' meaning to tread, from 'calx' (limestone, chalk). The same Latin root gives English words like calcium and chalk. Originally a nautical term referring to driving oakum into ship seams, it broadened over time to cover general sealing work in building and plumbing.
Related word forms
How caulk is actually used
Common in both everyday home-improvement contexts and professional trade usage; fully standard in both British and American English. The nautical sense is now largely historical but appears in writing about traditional boatbuilding.
Easily confused with caulk
Cork is a buoyant material from the bark of the cork oak tree or to stopper a bottle, while caulk is a sealant applied to joints and seams.