It is an Indian English term, probably from BrE old slag term tiffing, meaning take a little drink:

In the British Raj, where the British custom of afternoon tea was supplanted by the Indian practice of taking a light meal at that hour, it came to be called tiffin. It is derived from English colloquial or slang tiffing meaning to take a little drink, and had by 1867 become naturalised among Anglo-Indians in the north of the country to mean luncheon.

(Wikipedia)

It is an Indian English term, probably from BrE old slag term tiffing, meaning take a little drink:

In the British Raj, where the British custom of afternoon tea was supplanted by the Indian practice of taking a light meal at that hour, it came to be called tiffin. It is derived from English colloquial or slang tiffing meaning to take a little drink, and had by 1867 become naturalised among Anglo-Indians in the north of the country to mean luncheon.

(Wikipedia)

The ODO suggests that its origin is from a dialectal term meaning “sipping”:

Tiffin:

Early 19th century: apparently from dialect tiffing ‘sipping’, of unknown origin.