Lawd have mercy is the phonetic spelling of the expression "Lord have mercy". The pronunciation of the word "lawd" is a cultural colloquialism, historically
common within rural blacks in the South. It can still be
heard today especially among the older generations. "Lord have mercy on us" has origins from the Biblical scripture Psalm 123:3-4 & is usually expressed in times of trouble or worry. Also, a broader more generic usage can denote surprise or wonder.