A stick with a piece that fits under the arm, that you lean on for support if you have difficulty in walking because of a foot or leg injury. (Cambridge Dictionary)
Something that provides help and support and that you depend on, often too much. (Cambridge Dictionary)
A support typically fitting under the armpit for use by the disabled in walking. (Merriam – Webster)
A source or means of support or assistance that is relied on heavily or excessively. (Merriam – Webster)
A staff or support to assist a lame or infirm person in walking, now usually with a crosspiece at one end to fit under the armpit. (Dictionary.com)
How to Use Crutches. My Health Alberta.
Walking With One Crutch. Life Changing Medicine.
Crutches: 12 Tips for Going Up and Down Stairs.