Come to think of it, there are many more expressions like 조마조마하다. (가슴이) 두근두근하다(very common), 콩닥콩닥하다, 벌렁벌렁하다 all describe how the heart beats faster. In 조마조마하다, though, "조" and "마" are characters widely used in hanja (Chinese character) and not so much in Korean onomatopoeic words, to make it not so obvious that it is one.

불안하다: 불(不: not) + 안(安: save, secure) + 하다 => feel insecure and uneasy. * 불안하다 is the most common term for insecure, anxious, apprehensive, etc. 마음 졸이다: 졸이다 = boil down; shrink by heating => be fidgety, edgy, uneasy. * 졸이다 is evocative of the way the heart reacts, similar to "choking from fear". 조마조마하다: very nervous and fidgety from acute apprehension. * 조마조마하다 emphasizes the sense of "any second" nervousness. I think 조마조마 is an onomatopoeia describing the heart's agitated state of beating, although I couldn't find anything on the net about this. It is one of those strikingly vivid expressions in Korean.