It is a pseudo-verb. True verbs are conjugated according to number and person. For example, "to speak" is the infinitive. "I speak," where the verb is speak, conjugated as first person singular. In "he speaks," the verb is speaks, conjugated in third person singular. In "we speak" the verb is speak, conjugated in first person plural. There are three types of pseudo-verbs, infinitives, gerunds, and participles. Gerund form of to speak → speaking. Participle form of to speak → spoken.

Infinitive in Old English was declined as nouns. So, it had nominal qualities. In Old English texts it was registered in Dative singular, where it was used with the preposition 'to', e.g. to drincenne (Dative infinitive): Ic ga to drincenne 'I'm going to drink(ing)', like Russian 'питьё'. Eventually this preposition turned into the particle 'to'. Cf. drincan - 'to drink', which originally was Nominative case.

A noun.