In "Back to the Future", what did Marty say when things got difficult?

In “Back to the Future,” Marty McFly says, “Whoa, this is heavy!” when things get difficult or confusing.

The line is one of Marty’s recurring expressions throughout the 1985 film and its sequels. It works on two levels: “heavy” can mean emotionally serious, but Marty often hears it as a reference to physical weight. The joke becomes a running motif because Doc Brown repeatedly treats the phrase as something that may have a connection to gravity or mass.

The film’s time-travel story makes the line especially memorable. Marty travels from 1985 to 1955 in Doc’s modified DeLorean, where he meets his teenage parents and accidentally threatens his own future. His attempts to repair the timeline create situations that are literally and figuratively difficult.

The wording is sometimes written with punctuation as “Whoa, this is heavy,” but the stored answer identifies the quote correctly.