The idiom ‘under the weather’ is believed to have been around for quite some time but its oldest written account could be found in a newspaper in the year 1835. It means to feel sick, sad or depressed. The term has its origin in sailing. Sailors who used to fall sick while travelling would often go below the deck to get well and to protect themselves from bad weather. The term ‘under the weather’ probably shaped up due this. For example:
I have been feeling under the weather for quite some time now.