A pyrrhic foot is one with no stress.
A pyrrhic foot (also known as a dibrach) is one of the metrical feet used in English verse. You have probably heard of the iamb foot used in traditional iambic verse, consisting of an unstressed syllable followed by a stressed syllable, trochee, which has a stressed syllable followed by an unstressed one, and spondee consisting of two stressed syllables. As you might expect, phyrrhic meter completes the set, consisting of two unstressed syllables.
Obviously, a phyrrhic foot is not very exciting, and it isn’t used very often. Overuse would result in a monotonous and depressing poem. But pyrrhic feet are used in poetry in which lines are generally defined by the number of stressed syllables rather than the total number of syllables; this includes most, if not all, English verse. Adding in a pyrrhic foot buys you more words without adding to the number of stresses.
I do not have any examples of words that use pyrrhic feet in everyday speech, although ‘u-huh’ and ‘uh-uh’ might count. It is much more common to have multiple unaccented syllables in connected speech such as verse. Here is an excerpt from Shakespeare’s Twelfth Night, which includes a few instances of phyrrhic feet, outlined in italics.
That, notwithstanding thy capacity
Receiveth as the sea, nought enters there,
Of what validity and pitch soe'er,
But falls into abatement and low price,
Even in a minute: so full of shapes is fancy1.
The feet listed above are all duple meter (two syllable) feet. There are also triple meter feet, including dactyl, anapest, and amphibrach. Dactyl and anapest include two adjacent unstressed syllables, but they don’t count as phyrrhic feet. In triple meter a set of three unstressed syllables is known as a tribrach foot.
1. This last line has 13 syllables, so one might debate how to apply the meter.