Readings & Meanings:
side, width, beside
Since you're sick, you should lie down a little.
I put a fan next to the table.
We cross at the pedestrian crosswalk.
Stop acting lazy!
木 on the left. Think of a tall tree.
Then we have 黄 on the right! We just learned about 黄! (well, they're right next to each other because the 黄 radical is a phonetic marker, and thus lends the same sound to 横... but you can think it's just coincidence if you want to).
This time, don't think about the yellow flames coming off that arrow in 黄. Think about it flying through space. Yellow lines in space. Horizontal, you might say.
Bonus: Terrible Mnemonic
Remember John Lennon and Yoko Ono's "bed-in" for peace? Well, to stay in bed all that time, Yoko had to remain horizontal...
Sorry, but I think you'll remember 横 means horizontal, now.
Bonus: Better Mnemonic
横 is also a part of 横綱. The Yokozuna is the grand champion of sumo... except that there can be more than one at a time.
Here are the two current Yokozunas fighting each other... in a kind of anime style. (taken from the wonderful Watashi to Tokyo weblog--thanks Mari!)