GUIContent.tooltip
var tooltip: string;
string tooltip;
tooltip as string
Description

The tooltip of this element.

The tooltip associated with this content. Read GUItooltip to get the tooltip of the gui element the user is currently over.
	function OnGUI () {
		GUI.Button (Rect (0, 0, 100, 20), GUIContent ("A Button", "This is the tooltip"));
		// If the user hovers the mouse over the button, the global tooltip gets set
		GUI.Label (Rect (0, 40, 100, 40), GUI.tooltip);
	}
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnGUI() {
        GUI.Button(new Rect(0, 0, 100, 20), new GUIContent("A Button", "This is the tooltip"));
        GUI.Label(new Rect(0, 40, 100, 40), GUI.tooltip);
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnGUI() as void:
		GUI.Button(Rect(0, 0, 100, 20), GUIContent('A Button', 'This is the tooltip'))
		GUI.Label(Rect(0, 40, 100, 40), GUI.tooltip)