In Scrabble, words that cannot be played include those that are misspelled, made-up words, proper nouns, proper names, foreign words not in the dictionary used, abbreviations, acronyms, freestanding prefixes and suffixes, hyphenated words, and words with apostrophes. Additionally, a word cannot be played if the player lacks the required tiles to form it on their rack, such as when there are not enough of a specific letter tile available in the game set. For example, the word "pizzazz" cannot be played if there are not enough 'Z' tiles in the available pool, since Scrabble only includes two 'Z' tiles and "pizzazz" requires four.
Thus, which specific words cannot be played depends partly on the tile availability for that game and partly on whether the word fits the legal word rules of Scrabble.