What did Washington mean when he wrote that the maxim "honest is the best policy" should be applied to foreign relation?

"Honesty is the best policy" means that it is always best to tell the unvarnished truth. When we apply that to the area of foreign relations, it means that the interests of the US and their opinions on a foreign matter must be clear and said withouth any hessitation. Lying is the best way to lose credibility in the international game, therefore, all the officials must rely on "true" facts and those are the ones that will be the best way to reach the best solution.