The word "had" is the past tense form of the verb "have." It is used to indicate possession or experience in the past. For example, when someone says "I had a dog," they mean that at some point in the past, they owned a dog. Similarly, "I had a great time at the party" implies that the speaker experienced joy during the event in the past.
The word "had" is derived from the Old English "hæfdan," which means "to have." It has its roots in the Proto-Germanic "*habjan," and is related to similar words in other Germanic languages.