a coined
phrase used as a warning that one should be wary, careful or cautious in regards to something. It is a combination of the words
ware, which is used to tell someone to take
heed or beware of (something), to
look for and guard against and a shoal is a submerged sandbank that constitutes a hazard to
navigation. Ware shoal would literally mean to beware of a shoal (that could prove
dangerous to one's
ship).