This instance of onomatopoeia imitates the sound of sneezing. The first syllable mimics the quick intake of breath, while the second corresponds to the tone of the convulsive expulsion of air through the nose and mouth. “Achoo” is also considered an interjection, in the same class of words as “ouch” or “gosh.” (What are ”gosh,” “golly,” and “gee” short for? They all pertain to religion. Click here for the answer.)
Other languages follow the same approach. A sneeze sound in Russian is apchkhi; in Korean it is achee.
In the medical world, ACHOO is an acronym for a sternutation disorder called Autosomal Dominant Compelling Helioophthalmic Outburst Syndrome that results in uncontrollable sneezing.
As for the origin of “God bless you,” there are a number of ideas. There are superstitious beliefs that connect evil to sneezing, such as the thought that a sneeze releases a soul to the waiting grasp of evil spirits. Hence, a blessing is needed.
A false belief that originated during the Renaissance dictates that a sneeze causes the heart to momentarily stop. The blessing was a brief prayer that the heart would not fail completely.
There are numerous other tales that have to do with sneezing. For example, one folk saying asserts a sneeze means that someone is thinking amorously about you. What sneeze myths do you know about?