:BLT: /B-L-T/, /bl*t/ or (rarely) /belt/ n.,vt. Synonym for
{blit}. This is the original form of {blit} and the ancestor
of {bitblt}. It referred to any large bit-field copy or move
operation (one resource-intensive memory-shuffling operation done
on pre-paged versions of ITS, WAITS, and TOPS-10 was sardonically
referred to as `The Big BLT'). The jargon usage has outlasted the
{PDP-10} BLock Transfer instruction from which {BLT} derives;
nowadays, the assembler mnemonic {BLT} almost always means
`Branch if Less Than zero'.
-- The AI Hackers Dictionary