Difficult, as in Tom found calculus heavy going, or It's going to be heavy weather for us from here on. The first expression originally referred to a road or path that was hard to negotiate; the variant alludes to bad weather at sea.
make heavy weather of. Make hard work or a fuss over something, especially unnecessarily. For example, They made heavy weather of the differences between their proposals, which actually seemed much alike. This use of weather likens a commotion to a storm.