"Nomen agentis" is a grammatical term that means agent noun and refers to a noun created from a verb denoting an action, where the noun identifies an entity that performs that action. Some examples include kicker, swimmer, hiker, speaker.
The nomen agentis is the subject of the sentence.
In Latin, the nomen agentis fututor, meaning fucker, is derived from the verb "futuo, futuere, futui". Latin fututor has entered modern English as futterer, retaining the meaning of fucker.