Be chasing (one's) (own) tail American British verb phrase
Used when someone is busy to do a lot of things but achieve little and do not lead to progress
I have been chasing my tail all day, but I haven't finished any tasks.
Used when one wishes to engage in sex with someone
They admitted that they were chasing tail in the bar last night.
The verb "be" should be conjugated according to its tense.
Something or someone has been somewhere and become so familiar that it is hard for you to accept that place without them.
He was here for such a long time, so he was part of the furniture.