Why are BigInt chunks-of-bits called "limbs"?

Question

I've come across multiple sources that use the word "limb" to refer to a chunk of an arbitrary-sized integer. I can understand why "digit" and "word" aren't great names for this, but I'm curious where "limb" comes from, both in in terms of the history of the word and the reasons for choosing it over something else.

I've seen this:

• in the Zig standard library's std.math.big
• in the Rust crypto-bigint crate
• in the docs for BearSSL, which says "For non-decimal bases, it is traditional to call limbs the individual a_i values, intead of 'digits'."

I believe the author, but this is the first time I've seen it so I'm curious. Anyone know where the terminology originally comes from or why "limb" was chosen over other reasonable words?

Answer 1

According to the GNU MP manual 3.2 Nomenclature and Types...

A limb means the part of a multi-precision number that fits in a single machine word. (We chose this word because a limb of the human body is analogous to a digit, only larger, and containing several digits.) Normally a limb is 32 or 64 bits. The C data type for a limb is mp_limb_t.

This text dates back to at least May 1996.