Chaîné in modern French means chained or bound, and the origin of châiné in dance seems to come from very early social dances. What we know as chaîné (or chaîné déboulé) today may have been highly influenced by one of these social dances in particular.
Chaîné is pronounced [ʃɛne]. Ch- is pronounced like "shhh" in English. -ai is pronounced like "e" in dress. And -né - sounds like "nay" but without the final y sound being pronounced.
Chaîné is performed by taking two steps to complete one full revolution of a turn, each step resulting in a half turn, and generally continues for multiple revolutions. The speed of chaîné turns is highly variable and, as such, the turns are generally assigned a length of music (or time) rather than a number of steps. The steps should remain of equal size.
It is often helpful for the dancer to imagine that he/she is stepping on the links of a chain or that the ankles are chained together to achieve the short even steps necessary for a series of chaîné turns.
Looking backwards on chaîné turns, we find two entries in the 1787 Dictionnaire de danse defining chaîné as "contredanse," which is basically social dance performed in groups of 4s and 8s, and "Danse ancienne qui se faisait en tenant une corde, ou en se tenant par la main, ce qui faisait une sorte de chaîne." Imagine first a social dance where the participants change place, taking and releasing hands as they move weave through each other. If observed from above, the crossing of bodies would create the shape of a chain. The second entry translates to an "old dance that is done while holding a cord (or string) or holding hands which makes a sort of chain." The first part of this entry is quite interesting as one imagines either weaving or turning while holding a string. I have seen teachers introduce chaîné in exactly this fashion, asking dancers to hold one end of a string and to roll towards the fastened end and then unroll.