pi X en Y refers to a largely historical style of Toki Pona, in which en has lower priority than pi and may be used within pi phrases to join multiple modifiers or modifier phrases. It evokes the controversial glossing of pi as "of", which is proscribed and considered misleading in standard usage.
This style is noted in the "Notes on lipu pu" section of the Toki Pona Dictionary, which states that it "continues to be used by some people; however, many people dislike using it". Two examples are provided as follows:
linja pi(ma en sewi)
linja pi ma en sewi
horizon (line of land and sky)
musi pi(kiwen-walo en kiwen-pimeja)
musi pi kiwen walo en kiwen pimeja
chess (game of white stones and black stones)