An underdog is a person or group in a competition, usually in sports and creative works, who is largely expected to lose. The party, team, or individual expected to win is called the favorite or top dog. In the case where an underdog wins, the outcome is an upset. An "underdog bet" is a bet on the underdog or outsider for which the odds are generally higher.
In the battle between David and Goliath, David is an archetypal example of an underdog.
Goliath is a Philistine warrior in the Book of Samuel. Descriptions of Goliath's immense stature vary among biblical sources, with the Masoretic Text describing him as 9 feet 9 inches (2.97 m) tall. Goliath issued a challenge to the Israelites, daring them to send forth a champion to engage him in single combat; he was ultimately defeated by the young shepherd David, employing a sling and stone as a weapon. The narrative signified King Saul's unfitness to rule, as Saul himself should have fought for Israel.
David and Goliath, a color lithograph by Osmar Schindler (c. 1888)
David hoists the severed head of Goliath as illustrated by Gustave Doré (1866)
David with the Head of Goliath, circa 1635, by Andrea Vaccaro
Artist's rendition of Goliath's fall (18th century, Charles Errard the Younger).