A modern adjectival outgrowth the noun "shyster." The original term refers to someone who behaves in a disreputable or unethical
fashion. Thus a "sheisty"
person or behavior is one which appears overtly, even lewdly, unscrupulous.
The origin of "shyster" may be from the
German "Scheißer" meaning pooper (MW) or from the old use of "shy" meaning disreputable (OED).