means A soft, malleable paste — traditionally linseed oil and chalk — used to seal glass into window frames or fill gaps, hardening as it dries.

from From the French 'potée,' meaning the contents of a pot — originally a fine polishing powder made in a crucible. The word itself traces back to 'pot,' so putty is, quite literally, pot-stuff: whatever paste came out of the potter's vessel, later narrowing to the gluey window-sealer we know today.