The point in one's career in which being kept on the payroll, will costs less than firing and hiring someone to fill that same job. This is typically used to refer to engineers who've shipped a sizable amount of code making it an extreme pain to fire them and teach someone old code left by another author.
Jack: You went to Yale, you should start your own company.
Zeerek: Nah, as the lead engineer and part time PM on the Microsoft Bing team, I am slowly reaching the irreplaceablity tipping point of my career, in which I'll be able to show up, do minimal work, and laugh my way to the bank until I'm old and decrepit.
Jack: I don't know whether you're lazy or incredibly smart.
Zeerek: I like to think I'm both.
aka tenured software engineer
hahaha ^