As a noun: a respiratory disease in horses; as a verb: third-person singular of 'heave,' meaning to throw, lift, or breathe heavily.
From Old English 'hebban' meaning 'to lift' or 'to raise.' The disease sense evolved because the condition causes labored, heavy breathing. The verb has multiple meanings that developed from the basic concept of forceful upward movement.
The word 'heaves' perfectly shows semantic drift—it started as a simple physical action (lifting) and became a medical condition name because the condition looks like heavy, repeated lifting movements of the chest! Language follows what our eyes see.
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