A friend gave me what appears to be a slim line, 3" long, older stainless steel pocket knife?

How about spraying the knife with WD40 to unlock stiff joints. Wait a bit and then try again.

Here's something you might try. If it's a liner lock, turn it over and look at the front, where the blade joins at the handles. Look at the groove where the blade rests when closed.

There may be a small flat spring that locks the blade into place when it's open. Other than that, you could have a very stiff spring or froze mechanism.

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