What does infirm mean?

Infirm takes it's meaning from the dictionary. There is no definition of infirm in the Income Tax Act.  It's up to your qualified medical practitioner to determine if you are 'infirm' and to document that infirmity, by describing the nature, commencement and duration of said 'infirmity'

The Family Caregiver Amount requires your doctor document the infirmity of your dependent in writing and there are special requirements for documentation of infirmity for children under age 18. The letter should indicate that the child by reason of a mental or physical infirmity, is and is likely to be for a long-continued period of indefinite duration, dependent on others for significantly more assistance in attending to personal needs and care when compared to children of the same age.

The challenge will be persuading your dependents to ask their physician to document their infirmity. If you thought that labelling someone as disabled was difficult, try explaining to your aging parent, a dependent sibling, or your teenage child that you would like them to ask their doctor to write such a letter declaring them to be infirm, all so you can pay $300 less tax per person. Good luck on that.

And yes, if you have the disability tax credit, that's not the same as infirm. Disability is actually worded and defined in the Income Tax Act as an 'impairment in ability'. You could have an impairment in ability but that may not mean you meet the criteria of infirm.  You for example, might be blind or hearing impaired, or you may be confined to a wheelchair, but definitely that doesn't necessarily make you infirm, given that the definition of infirm is weak or feeble. In the case of the wheelchair, you could be an incredible athlete, winning trophies at the Olympics. Are you considered infirm?  I guess only your doctor knows for sure.