Kalos kagathos (Ancient Greek καλὸς κἀγαθός [kalos kaːɡatʰǒs]), of which kalokagathia (καλοκαγαθία) is the derived noun, is a phrase used by classical Greek writers to describe an ideal of personal conduct, especially in a military context. Its use is attested since Herodotus and the classical period. The phrase is adjectival, composed of two adjectives, καλός ("beautiful") and ἀγαθός ("good" or "virtuous"), the second of which is combined by crasis with καί "and" to form κἀγαθός. Werner Jaeger summarizes it as ”the chivalrous ideal of the complete human personality, harmonious in mind and body, foursquare in battle and speech, song and action”. 
The phrase could be used both in a generic sense, or with certain specific force. As a generic term, it may have been used as the combination of distinct virtues, which we might translate as "handsome and brave", or the intersection of the two words "good" or "upstanding". Translations such as "gentleman" or "knight" have traditionally been suggested to convey the social aspect of the phrase, while "war hero" or the more cynical "martyr" are more recent versions, and emphasise the military element.
It became a fixed phrase by which the Athenian aristocracy referred to itself; in the ethical philosophers, the first of whom were Athenian gentlemen, the term came to mean the ideal or perfect man.
The possession of the beautiful and the good ("kalos kai agathos") has a correspondent in Latin: "mens sana in corpore sano" (healthy soul in healthy body). It is also seen as a target in balanced education of body and spirit.
There is thematic discussion of kalokagathia in Aristotle's Eudemian Ethics Book VIII, chapter 3 (1248b). And how a kaloskagathos (gentleman) should live is also discussed at length in Xenophon's Socratic dialogues, especially the Oeconomicus.
The adjective καλός means beautiful and encompasses meanings equivalent to English "good", "noble", and "handsome". The form given by convention is the masculine, but it was equally used of women (the feminine form is καλή) and could also describe animals or inanimate objects.
Plato, in his work Republic, used the term τό καλόν (the neutral form) in his attempts to define ideals. However, his protagonist in the dialogue, Socrates, stated that he did not fully comprehend the nature of this καλόν.
This second adjective means good and had no particular physical or aesthetic connotations, but could describe a person's excellence of character (ethical virtue) for example their bravery. Again, around the 4th century, it had become politically meaningful, and carried implications of dutiful citizenship.
- Werner Jaeger (trans. Gilbert Highet). Paideia, The Ideals of Greek Culture. New York: Oxford University Press, 1945.