The term "wage slave" is used seriously.
Working with consent for an agreed-upon wage (even if without it you would be oppressed by nature) is the opposite of being held against your will and compelled to work for whatever minimal sustenance your "owner" provides. Something cannot be both voluntary and involuntary. Thus "wage slavery" is an anti-concept, a contradiction (at least as used by communists, rather than tongue in cheek). (DBR)