Language Is Messy

A nice analysis of the origin of the sense of the phase "sleep in" in American English with the meaning "to sleep later than usual", nicely illustrates that point that language is messy. The first attested historical use of the phase in this sense in American English appears to have migrated via the older Scotish usage in or after a fiction reference (involving a Scotish character) in 1931 that somehow migrated from fiction to common usage.

A word that has a particular meaning on one phrase may have a different meaning in another context. Even the same phrase may have different meanings in different contexts in a single living language at a single historical moment.

This matters because, for a great many purposes in historical linguistics, it is counterfactually assumed for purposes of practicality of analysis, that words have only one semantic meaning which may be shaded a bit over time, but doesn't make the leaps of meaning that linguistics have observed in lots of common words in real life, relatively modern history.

Remembering that reality is messier than it is assumed to be is worthwhile, even when one does use simplifying assumptions out of necessity. Otherwise, one might fail to take one's own results with the grain of salt that these results absolutely require.