A variation of the word "hey." Can be used as a greeting, usually with the syllable drawn out while simultaneously rising and falling in inflection, as illustrated below in Example 1.
Can also be used as an expression of enthusiastic agreement in a conversation, as demonstrated below in Example 2.
One theory suggests that this variation of "hey" was popularized by the character of Dwayne (portrayed by Haywood Nelson) in the sitcom "What's Happenin'," although Dwayne's usage was three "hays" with shortened syllables, with the inflection on the middle "hay," as shown in Example 3 below.
Person #1: "It sure is hot today!"
Person #2: "HaAAAAAay!"
"Hay HAY hay!"