Lou Adler claimed at the trial that the safety deposit box was for stock investments that Dean and Barry Keenan had been making together. They'd been friends since their high school days.
$25,000 was a LOT of money in 1963--back then it would buy either one new Ferrari 275GTB or about five new Corvette Sting Rays with all the options!
I wouldn't really classify Dean as a badass. He claimed he gave Keenan that money simply because he was unemployed; he didn't believe the guy would actually kidnap Frank Sinatra, Jr., even after he'd told Dean of his plan. Apparently, Dean believed this was just another of a series of practical jokes they'd shared together, or just a silly way of asking him for money. He claimed he thought Keenan was joking when he told him about his scheme.