A foolish, vapouring gossip. The Scotch say, “Haud your tongue, maundrel.” As a verb it means to babble, to prate. In some parts of Scotland the talk of persons in delirium, in sleep, and in intoxication is called maundrel. The term is from Sir John Mandeville, the traveller, who published an account of his travels, full of idle gossip and most improbable events.
There is another verb, maunder (to mutter, to vapour, or wander in one's talk). This verb is from maund (to beg). (See Maundy Thursday.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894