From Wiktionary, the free dictionary
From xanh (“grue”) + lè; the second element here is probably the same as in chua lè (“very sour”), cay lè (“very spicy”).
xanh lè
- very green in a bad way
- Coordinate terms: đen thui, đỏ lòm, trắng toát, vàng khè, xám xịt