Initialism of magnetic resonance spectroscopy. / Initialism of mandibular repositioning splint. / Initialism of marginal rate of substitution. / Initialism of metabolic risk score.

(UK, rail transport) Initialism of Midland Railway.

Clipping of MRI. / (pharmacology, medicine) Initialism of modified release. / (cardiology) Initialism of mitral regurgitation. / (biochemistry) Initialism of mineralocorticoid receptor.

(education, special education) Initialism of mentally retarded.

(folklore) The wife of Santa Claus.

(US, politics) Nonstandard form of Mr. President. / Alternative form of Mister President

Alternative form of Mr. Big

(informal) The head of a crime syndicate, a high-ranking criminal.

plural of Mr Big

(Britain, informal) A charlady.