jQuery.asuggest is a small light weight plugin that can be used to implement auto-complete with textareas. This plugin requires the following two js

jQuery.asuggest can be downloaded from here or from its GitHub page. Lets quickly dive into an example :

Html : (jsFiddle link)

<textarea rows="10" cols="55" id="comment"></textarea>

Jquery : (jsFiddle link)

$(document).ready(function(){

    var suggests = ["hello", "android", "world", "is", "all", "over", "under"];
    $("#comment").asuggest(suggests);    

});

Output : http://jsfiddle.net/yrshaikh/JbYdx/embedded/result/
jquery asuggest output

More examples