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1.
The weight that an individual gains after starting a new relationship due to the large(or not) volume of semen that is consumed during foreplay and/or sexual intercourse.

Can be used in a derogatory manner or not depending on individuals and context of conversation.
Non derogatory:
I have put on 6lbs since dating him, its totally cum weight.

Derogatory:
That bitch is so fat, its totally all cum weight.
by NMasters September 25, 2012