fun sponge

Suggesting that such a person, etc. would suck up the fun as a sponge sucks up water.

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fun sponge (plural fun sponges)

1. (figurative, informal, mildly derogatory, usually endearing) Someone who or something that takes the fun out of life; a killjoy.

• buzzkill, killjoy, spoilsport; see also Thesaurus:spoilsport