Why custard doesn't cut the mustard

Last week I came across an expression I hadn't heard for years, courtesy of the Bangkok Post's cryptic crossword. The clue was "It's sweet (but cowardly)". The answer turned out to be "custard". That took me back to pre-teen days when "cowardy, cowardy custard" (without the 'L') was a taunt heard at my primary school when someone timid was being teased.