Throbbing; pulsing with a steady beat or vibration; in a state of rhythmic pounding.
From a- (prefix meaning 'in' or 'on') plus throb. This archaic or poetic form emphasizes the continuous state of throbbing motion.
Words like 'athrob' are poetry waiting to happen—the 'a-' prefix makes the word feel like something is deeply, persistently pulsing rather than just throbbing, which is why you find it in romantic or dramatic literature from the 1800s.
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