I think it originally must have been 'happy (you'), because it's a pronominal form of the second (like 'you') plural person of the imperative for the verb to rejoice; although it is also used for the second singular, changing accentuation. And of course, it can also be a vulgar pronunciation of the adjective or glad participle. See verbs/alegrad , verbs/alegra .
* Only one "like" per meaning and day, the more "likes" the meaning will appear higher in the list
