Soap is something that is used with water to make suds and is used for cleaning.
An example of soap is the bar sitting next to a bathroom sink used for washing hands.
See soap in Webster's New World College Dictionary
Origin: ME sope < OE sape, akin to Ger seife < Gmc *saipo- < IE base *seib-, to trickle, run out > L sebum, tallow
See soap in American Heritage Dictionary 4
Origin: Middle English sope
Origin: , from Old English sāpe.
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