(A), in workman's language, means a portion of time
allotted to some particular work, and from which the men are relieved
when the limited time expires.
is to relieve another at his work.
An exclamation to signify that the allotted time has expired, and
men are to be relieved by another set.
A pretty good spell.
A long bout or pull, as a “spell at the capstan,” etc. (The German spicl means a performance as well as a play, game, or sport.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894