Select the INCORRECTLY spelt word.

To find the incorrect spelling, let us examine each of the given options one by one:

1. Dictionary: This is spelt correctly. A dictionary is a book or electronic resource that lists the words of a language and gives their meaning.
2. Reference: This is spelt correctly. Reference refers to the action of mentioning or alluding to something.
3. Infurmary: This is spelt incorrectly. The correct spelling is Infirmary (with an 'i' instead of the second 'u'). An infirmary is a place in a large institution (like a school or college) for the care of those who are ill.
4. Ordinary: This is spelt correctly. Ordinary means with no special or distinctive features; normal.

Therefore, the incorrectly spelt word among the choices is "Infurmary".