la·tis·si·mus dor·si 
(lə-tĭsə-məs dôrsī)
(lə-tĭsə-məs dôrsī)n. pl. la·tis·si·mi dorsi (-mī′)
Either of two broad, flat, triangular muscles with origin from the spinous processes of the lower thoracic and lumbar vertebrae, the sacrum, and the iliac crest, that insert into the humerus and whose action adducts, medially rotates, and extends the arm.
[New Latin lātissimus dorsī : Latin lātissimus, superlative of lātus, wide + Latin dorsī, genitive of dorsum, back.]