slip away is a verb meaning "leave furtively and stealthily". It carries 2 recorded senses.
leave furtively and stealthily
“The lecture was boring and many students slipped out when the instructor turned towards the blackboard”
pass by
“three years elapsed”
Definitions, synonyms and semantic relations for slip away come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.