With that in mind, layering is the key to dressing for Malaysia — specifically long, loose, lightweight clothing as it's culturally respectful and will keep you cool in the tropical climate. A light fabric like linen is great for the heat as it absorbs moisture (read: sweat) and dries quickly. You might want to avoid cotton as it takes ages to dry — not ideal for places like Borneo where the humidity can be oppressive! Sportswear is also a good option for hot weather.
Short shorts, tank tops, crop tops and spaghetti straps aren't culturally appropriate in Malaysia. While women are expected to dress more conservatively than men, men should also cover up from the shoulders to below the knee. The exception is when you're on a beach or by a pool, in which case wearing a bathing suit is fine. And remember: sarongs are not acceptable outside the beach or pool area.
You'll also need a lightweight raincoat or poncho. Malaysia has two monsoons — the Northeast Monsoon (November to March) and the Southwest Monsoon (May to September) — however, these patterns can be unpredictable and some places, such as Kuala Lumpur, experience almost daily showers. In Borneo rain is always a possibility. Downpours tend to be heavy but brief, so it's not a massive deal if you get wet; it's just not very comfortable walking around in damp clothes.
In terms of footwear, comfortable shoes/runners with good grip for walking are essential. You'll need hiking boots if you plan to hike Mt Kinabalu as the terrain is rocky, uneven and steep. We also recommend bringing a pair of sandals, which are handy for milling around the hotel and easy to remove at religious sites.
Wearing skimpy clothing is considered disrespectful to local culture in many of the small communities we visit. As a general guide, shoulders, cleavage and knees should be covered when travelling in Malaysia — especially outside of major cities. A loose-fitting skirt or trousers will also keep you cool and protect your skin from the sun and mosquitoes.
As a liberal Muslim country, many Malaysian women don't wear headscarves, and female travellers aren't expected to cover their hair. The exception is if you're visiting a mosque, in which case you'll need to bring a scarf to wrap around your head. You can usually rent a shawl or robe at major mosques, too.