Meum and Tuum
That which belongs to me and that which is another's. Meum is Latin for “what is mine,” and tuum is Latin for “what is thine.” If a man is said not to know the difference between meum and tuum, it is a polite way of saying he is a thief.
“Meum est propos'itum in taberna mari. ” A famous drinking song by Walter Mapes, who died in 1210.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894