Good question, Doug. Maybe it started as a contraction of "thumb is up."
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On then, to another peeve of mine, and that's the use of the word "existential" merely to mean "actual." For example, referring to a living, breathing zebra as an existential zebra. I suppose in contrast to a metaphorical zebra, whatever that may be.

When I was in college, existentialism was a sort of philosophy, a philosophy I never actually understood. No wonder. The Stanford Encyclopedia of Philosophy says:

... it is difficult to explain what the term “existentialism” refers to. The word, first introduced by Marcel in 1943, is certainly not a reference to a coherent system or philosophical school.[1] Indeed, the major contributors are anything but systematic and have widely divergent views, and of these, only Sartre and Beauvoir explicitly self-identified as “existentialists.”

Reading about it now, I realize that it was a the attempt of people bewildered and horrified by the atrocities of World War II to make sense out of human life.