Meaning of “Chickiru(チキる)”

“Chikiru(チキる)” is an abstraction for “chicken ni naru(チキンになる)” which means become coward.
In this case, chicken means coward and “ni naru(に なる)” means become.
This is a slang,

So if someone says “Chickirunayo(チキるなよ)”, it means “Don’t be a pussy”, kind of.
“Chickitteru(チキってる)” means same with “Chikiru(チキる)” and “Chickitteta(チキってた)” is the past tense.

Example Conversation (They are playing a online game)

A: Gogogogo! “Chickirunayo(チキるなよ)”!
B: I know!
After 30 min…
A: You “Chickitteta(チキってた)”.
B: Yeah, “Chickitteta(チキってた)”… sorry…

A: いけいけいけいけ！チキるなよ！
B: わかってるよ！
30秒後
A: うわー、チキってたねー。
B: チキったわ、ごめん。