the.com/buzzkill

means Someone or something that drains the fun or excitement out of a moment.

from A straightforward compound from American slang, pairing 'buzz' (the pleasant high or excitement of a good time, itself from the mid-20th-century slang sense of 'buzz' as a mild intoxication) with 'kill' (to stop or destroy). It surfaced in casual speech around the 1970s, riding alongside other drug- and party-culture slang, and stuck because it does exactly what it describes — kills the metaphor dead with blunt clarity.