I used this word as a wwftd not long ago; the OED citations I quoted were from Jennings himself, which put a date on the coinage:

"Resistentialism is a philosophy of tragic grandeur...
Resistentialism is the philosophy of what Things
think about us."
- P. Jennings, The Spectator 23 Apr. 1948

"Things are against us. This is the nearest English
translation I can find for the basic concept of
Resistentialism."
- P. Jennings, Report on Resistentialism (1950)

"I must object to Oxford's dubbing resistentialism a
"mock philosophy." There is nothing mock or sham about
it, as anyone who has ever had to call a plumber on a
Sunday morning to unclog a refractory toilet will
attest." - Charles Elster,
Resistentialism: Things that go totally awry