A disparaging name for the form of Christian religious observance which is informal, musical and spontaneous.
The term came into use in the 1990s in the UK. It is applied especially to aficionados of the Toronto Blessing form of worship. It seems to have originated in South Africa. The earliest citation I can find of the term in print is from the South African author, Rian Malan's My Traitor's Heart, 1990:
"Both were members of the Apostolic Church, happyclappies in South African slang - into the laying on of hands, faith healing, and speaking in tongues."