If we're counting only programming languages (and not fill-out forms), Prolog had it in 1971, before ML. ASCII didn't include _ until its 1963 draft, so it's probably somewhere in that time.

I'll guess it's probably Prolog, but maybe Planner (Prolog's predecessor) had it too.

ASCII 1963 was the first version of the standard and it lacked _ https://dl.acm.org/doi/10.1145/366707.367524

The 1965 draft had _ https://dl.acm.org/doi/10.1145/363831.363839

The first standard edition with _ was 1968 https://www.rfc-editor.org/info/rfc20

The 1977 version is also available https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/fipspub1-2-197...

I think this predates computers entirely.

I think so, yes. English novels written in the 1800s would reference "Mr A------ from ------shire". As if they were redacting personally identifying information. (1)

Underscores and dashes are not that different - especially as this is not just pre-computer but pre-typewriter.

And now, underscores are a logical choice when the dash is already in use as a minus sign.

1) https://literature.stackexchange.com/questions/1944/why-are-...

https://forums.welltrainedmind.com/topic/141704-jane-austens...

Underscore has been used in programming for the first time by the language IBM PL/I, in 1964-12, where it replaced the hyphen/minus character that was used by COBOL to make more readable the long identifiers.

Replacing hyphen/minus with underscore has been done precisely for removing the ambiguity with the minus operator (In COBOL the ambiguity was less annoying, because the arithmetic operations were usually written with words, e.g. ADD, SUBTRACT, MULTIPLY and so on).

Good point, this is clearly something that could go back centuries.. if not all the way back to clay tablets.