means To crush or pound something into a soft, pulpy mass — or, in slang, to flirt aggressively or press buttons frantically.

from From Old English 'māsc' or 'māx,' meaning the mixture of malt and hot water used in brewing — a sense still alive when you 'mash' grain to make beer. The crushing-into-pulp meaning grew from that brewing softness. The flirtatious 'mash' (and 'masher,' the showy ladies' man) bloomed in 19th-century theatrical slang, of murky origin — possibly the same word, possibly a separate thread; etymologists honestly aren't sure how the potato and the wooer ended up sharing a name.