같지않다 is not the opposite of 같다?

같다 is an adjective that has several meanings, including the following:

1. the same; identical (with)
2. equal (to); uniform; equivalent (to)
3. like; alike; as
4. seem; appear
5. worthy of; becoming

One might expect that 같지않다, abbreviated to 같잖다, would be the negative of 같다 and mean something like "not the same," but 같잖다 is defined as follows:

1. foolish; silly; absurd; improper; bothersome
2. trifle; trivial; small; slight; insignificant

Here are some example sentences using 같잖다:

• 같잖은 물건
  a worthless object
• 같잖은 녀석
  a good-for-nothing fellow
• 같잖은 일로 화를 내다
  get angry at trifles
• 같잖은 소리를 하다
  talk impudently (to); give (a person) cheek (some lip)

So, my question is this: If I were to say, 이것과 그것이 같잖다, would I be saying, "This and that are worthless," or "This and that are not the same"?