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To be on a mad one=to be going crazily angry at no apparent provocation
Seán: Can I have a pencil please Miss?
Miss Griffin: No Seán, go and buy your own f*cking pencils, you little b*ll*cks.

Seán(to friends):Jesus lads, don't go near Griffin today, she's on a mad one.
by Jamachine May 08, 2007
English phrase used to describe the state an individual or party of people are in when they have ingested a significant amount of any one or combination of intoxicating substances and have aspirations of engaging in sexual relations and/or violent conflict with other rindividuals
I am on the mad one sir
by Tom Rowsell May 30, 2007