Literally, 腰掛け means just “a seat”, however, this word is most often used to refer to a temporary job while looking for a better, long-term job.
腰掛け has been used this way since Edo period. When we say “seat” though, we’re not referring to actual furniture, like the word 椅子, or “chair.” Instead 腰掛け has a connotation of a seat that only used for a temporary rest. This is how we get to the meaning we use today - 腰掛け is a metaphor for “a temporary position” while looking for a job that you truly want.
腰掛け is also has a side meaning to refer to an employee who works just to kill time. Until recently, female employees were considered as as 腰掛け as many women left their job once they got married. In recent years, women tend to stay much longer at their jobs and the idea that female workers are a type of 腰掛け has become much weaker. Still, you can hear the word used in Japanese TV shows sometimes (such as in the popular Netflix show, “Aggretsuko”!).
腰掛け is a noun, and usually has a negative connotation. If you call someone a 腰掛け, it means you do not think that he or she is not serious about their job. So, please be careful when using it!
Nursing cannot be a job for people who are just looking to work to kill time.
Because he is so smart, this job would likely just be a temporary position for Joe.