'Ass-mar' is a phonetic spelling of the respiratory condition asthma. If I am not mistaken this is from the novel 'The Lord of the Flies'. The character Piggy is complaining that he can't breathe properly because of his asthma and the character Ralph replies 'sucks to your ass-mar'.
It's an extremely vulgar way to refer to a very promiscuous woman. It's not an actual word, is the combination of the words "ass" (culo) and "loose" (suelto/a), and those words together mean a woman with a loose ass. I highly recommend not to use it.