Abnormally is from abnormal plus -ly. The suffix -ly recasts abnormal as an adverb: the lexical idea remains recognizable, but the word now describes manner, degree, or circumstance rather than naming the quality by itself. In use as an adverb, abnormally denotes in a way that is different from what is normal or expected. Typical contexts include “The patient's heart was beating abnormally fast”. The meaning stays close to the form: it applies the etymological material to in a way that is different from what is normal or expected, keeping the visible parts legible in ordinary use.