Rare synonym for 手首 (てくび). Mostly encountered in older texts or as a variant; modern speakers use 手首.
See also: 手首
In old documents, 腕首 is sometimes used to represent うでくび.
The word うでくび is hardly used nowadays.
Compound of 腕 (うで, 'arm') and 首 (くび, 'neck'), literally 'arm neck'. The exact historical derivation is uncertain; it is a rare synonym for 手首.