The term 'plug up' can be defined as a phrase meaning to STOP, FILL, and CLOG. The crossword solutions for this term are STOP (4 letters), FILL (4 letters), and CLOG (4 letters). To plug up something is to block it off, usually with a material such as a cork or a stopper. It can also mean to FILL a hole or gap with a material such as cement or putty. Finally, it can mean to CLOG something, such as a drain or pipe, with an obstruction.