-mu Godan/u-verb

1. to be crowded; to be packed; to be congested; to be thronged (with) (usu. 混む)
2. to be complex; to be intricate

suffix

3. to go into; to go in; to put into (after -masu stem of verb)
4. to become (completely) (after -masu stem of verb)
5. to do thoroughly; to do sufficiently (after -masu stem of verb)
6. to remain (silent, seated, etc.); to stay ... (after -masu stem of verb)