Yes, COYEST is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, Y 4, E 1, S 1, T 1.

Is it a valid word?

COYEST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

affectedly modest or shy especially in a playful or provocative way
showing marked and often playful or irritating evasiveness or reluctance to make a definite or committing statement
modestly or warily rejecting approaches or overtures