The word "smug" originated in the mid-1500s and likely comes from Low German "smuk," meaning "neat, trim, spruce."  It's possibly related to Middle Low German "smücken," meaning "to adorn" or "to dress". The sense of smug as being self-satisfied or conceited developed later, possibly influenced by the idea of being well-groomed or neatly dressed. 

The word "smug" first appeared in the English language in the mid-16th century, around the 1550s. Initially, it meant "trim," "neat," or "well-groomed". The current meaning of "excessively pleased with oneself" developed later, around the early 18th century.  According to the OED, smug first appeared as an adjective: 1551 in a translation by Ralph Robinson; as a verb: 1588 in the writing of Robert Greene, and the earliest known use of the noun "smug" is in the early 1600s, with OED's earliest evidence from around 1600-9. 

Examples of "smug" used in a sentence: 
"She deserved her promotion, but I wish she wasn't so smug about it."
"There was a hint of smug self-satisfaction in her voice."
"He's been unbearably smug since he gave up smoking." 
"He folded his hands over his chest and gave her a smug smile." 
"Fred leaned back, as smug as a raffle winner." 
"Sasha sat on top of the sarcophagus and looked around, smug in how safe he was sitting on top of the coffin."
"Fred sorted his notes, a smug look on his face."
"Victor's smug laugh stole Jackson's breath, So touching, these feelings you have for a werewolf."

smugness comes before a fall 

Just the facts:

SMUG Definition & Meaning - Merriam-Webster