The Mother Verb

Once my Spanish professor asked the class what 'el verbo infinitivo' meant. So I raised my hand and say "The infinitive form of the verb.", real smart-like. He tells me that is the literal translation, but that he want to know what it means. So I say, "It's the mother verb". He stares at me for a few seconds like I am some alien just come aboard this planet, then laughs awkwardly. Now he knows me as the 'mother verb' girl... Oh joy.