The word "shark" entered the English language in the 1560s, well after professional lawyers emerged in the thirteenth century. The term "shark lawyer" has been used to describe lawyers for centuries and has multiple origins:
In US: must be an attorney licensed and in good standing in any state, territory or DC.
Outside US: must be a lawyer or equivalent (eg counselor, barrister, advocate, solicitor), duly educated and licensed/accredited and in good standing.
As a general rule, experienced and currently practicing lawyers, and those teaching law in the legal academy, are more likely to be admitted.