Originally from the expression dat ass
, in which the targeted female derriere is said to induce an emotional response of amazement and/or attraction.
The same principle applies, except the 'ass' is replaced by whatever object is source of the impression. Plural version can also be used ("dem ...", see Example 3).
Can also be used in a sarcastic way with respect to the above utilization. (See Example 4, 5)