In MIDAS (the assembler used on ITS), the mnemonics for the system calls (UUOs, actually) all started with “.”: .OPEN, etc.
.LUNCH? might be taken as a combination of assembly language and Muddle, since Muddle’s predicates all ended with ? (vs. LISP, where they ended with “p”). It meant, “time to get lunch?”
-ta
This explanation from Tim Anderson: