Similar word to phlegm:

aloofness, coldness, coolness, detachment, disinterest, dispassion, disregard, dullness, emotionlessness, halfheartedness, heedlessness, impassivity, incuriosity, incuriousness, indifference, insensibility, insensibleness, insensitivity, insouciance, lassitude, lethargy, listlessness, passiveness, passivity, stoicism, stolidity, stolidness, unconcern, uninterest, unresponsiveness