Generally used to refer to the smallest denominated Philippine coin. Presently, and for most of the time in the past, that is the Five Centavo coin.
There have been times when the Bangko Sentral ng Pilipinas (Central Bank of the Philippines) have minted One Centavo coins. During those times, “Kusing” meant one centavo.
“Kusing” is often used in an idiomatic expression to imply having no money at all: “Wala ako kahit ‘sang kusing” (I’m penniless).
“Kusing” appears to be a bastardization of the Spanish “cinco” (English: Five), which is how the 5-centavo coin is called in most Philippine vernaculars. [i.e., from cin-co to co-cin].
Trivia: “Kusing” was used in the Magandang Balita Biblia (Good News Bible) as the Tagalog translation of the word “mite” in Mark 12:42 which reads (in English, NKJV): “Then one poor widow came and threw in two mites (dalawang kusing), which make a quadrans (isang pera).”