Webster's Revised Unabridged Dictionary · 1913

Nag

Nag (nag) , noun

[Old English nagge, Dutch negge; akin to English neigh.]

1.
A small horse; a pony; hence, any horse, especially one that is of inferior breeding or useless.
2.
A paramour; -- in contempt. [Obsolete] — Shakespeare

Nag , verb, transitive and intransitive

[Compare Swedish nagga to nibble, peck, Danish nage to gnaw, Icelandic naga, gnaga, German nagen, & English gnaw.]

To tease in a petty way; to scold habitually; to annoy; to fret pertinaciously. [Colloquial]
She never nagged. — J. Ingelow

Nag (nag) , noun

A person who nags, especially habitually; called also nagger.