I believe 'estoppel' is a legal term for a motion to have declared in law what is actually the case in fact. e.g. a lawyer employs questionable tactics to make money for his law firm where he is not a partner. Law firm takes all the money he generates him and berates him about his behaviour. However, it keeps taking the money up to the point where his contribution outweighs that of the Partners of the Law Firm. They fire him for cause (doing something questionable). He launches an estoppel claim to have himself declared a Partner of the firm so as to obtain his share of the profits which is in fact, all of the profit. He is not a partner in legal terms, he is/was an employee, but the estoppel claim is lodged to have declared in law that he is, in fact, a partner by dint of his financial contribution to the business. That's in the United States of course, it may have a subtlety different meaning here as is the case with many legal terms.