自嗨

zìhāi
  1. (slang) to have fun by oneself
  2. to amuse oneself

CEDICT

slang

Entry Composition

1 自 zì
  1. (bound form) self; oneself
  2. from; since
  3. naturally; as a matter of course
2 嗨 hāi
  1. oh alas
  2. hey!
  3. hi! (loanword)
  4. a high (natural or drug-induced) (loanword)