From Middle Dutch wac. Related to wake, from Old Dutch *waka, from Proto-Germanic *wakwō. Probably not identical to this form, however, as both the gender and formation are different: wake and its cognates are feminine ō-stems, while wac is a neuter a-stem. It would therefore have to derive from Proto-Germanic *wakwą, but this form has no other known descendants.
Cognate with Middle Low German wake (German Wake), Old Norse vǫk (Icelandic vök, Swedish vak).