Calling for a Huey

In Viet Nam, soldiers called for a Huey helicopter to evacuate. The phrase "calling for a Huey" became a euphemism for evacuation of the GI tract (vomiting).

This is onomatopoeia, I believe. I believe it's equivalent to "saying 'Huuuuey'", imitating a retching sound. A more common expression along the same line, I think, is "selling [or buying, etc.] Buicks", i.e., making a "buuuuick" sound.