This challenge takes in text (ie thisistext), combines each two letters pairwise (th is is te xt), gets their combined encoding (0x7468 0x2069 0x7320 0x6973 0x2074 0x6520 0x7874), and gets that nth word from the dictionary. As it turns out, that's mostly the ds of this particular dictionary.
It can be solved by taking each word, finding its position in the dictionary, taking the 0xff00 mask for the first letter the word represents, and the mask 0x00ff for the second letter.
Flag - byuctf{what_an_inefficient_code_ug2Ko8Cz}