fly-by-night

A fly-by-night is defined as an unreliable person who is generally not known and who cannot be trusted.

(noun)

An example of a fly-by-night is a contractor who takes your money to produce your kitchen cabinets and then never produces them.

The definition of a fly-by-night is an unreliable person who is generally not known and who cannot be trusted.

(adjective)

An example of fly-by-night person is the description used for a salesman who promises the world but doesn't deliver.

See fly-by-night in Webster's New World College Dictionary

adjective

not trustworthy; esp., financially irresponsible

noun

a fly-by-night person; esp., a debtor who runs away to escape debts

See fly-by-night in American Heritage Dictionary 4

adjective

Unreliable or unscrupulous, especially with regard to business dealings: “fly-by-night telephone companies that open up shop, sell some systems, then disappear when service is needed” (Mary Ellen Jordan).
Of an impermanent or insubstantial nature: fly-by-night fashions in clothing.

noun also fly-by-night·er (-nīˈtər)