A dictionary (used to look up words or kanji readings and meanings). An older or colloquial term equivalent to 辞書.
漢字の書き方が分からず、字引を引いた。
I consulted a dictionary because I didn't know how to write the kanji.
図書館で古い字引を見つけて読みふけった。
I found an old dictionary at the library and got absorbed in reading it.
分からない語は、まず字引で調べてみよう。
If you don't know a word, first look it up in a dictionary.